impl(gax-internal): support server-side streaming

[suzmue]

Add the execute_server_streaming helper on the gRPC transport client to support server-side streaming RPCs. The helper handles converting domain requests to proto messages and decodes the response stream using decode_response_stream into a ResponseReceiver.

Gate server streaming execution helpers under the
google_cloud_unstable_gapic_streaming cfg.

Enable include_server_streaming_methods for Showcase in librarian.yaml, regenerate google-cloud-showcase-v1beta1, and add integration tests for server-side streaming.

For #2318

[gemini-code-assist]

Code Review

This pull request adds support for server-side streaming RPCs in the Rust SDK. It implements execute_server_streaming in the internal gRPC client, adds unimplemented server streaming stubs, and updates the generated showcase client, builders, and transport layers to support streaming methods like expand, stream_blurbs, and attempt_streaming_sequence. Comprehensive integration and unit tests are also included. The review feedback recommends using the fully qualified path google_cloud_gax::streaming::decode_response_stream in src/gax-internal/src/grpc.rs to ensure consistency with the fully qualified ResponseReceiver type used nearby.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.40%. Comparing base (2b3004b) to head (294eabb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6504   +/-   ##
=======================================
  Coverage   96.39%   96.40%           
=======================================
  Files         301      301           
  Lines       84769    84807   +38     
=======================================
+ Hits        81714    81755   +41     
+ Misses       3055     3052    -3     

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[coryan]

Very neat. I think you may be making progress towards some of these: #2202, #2204 #2205, #2206 and may have even fixed them. If you did, please close the issues.